arrangements are in progress and must be executed with due diligence, provided that it is necessary to ensure successful removal. 65 It must therefore be apparent, at the time of the national court’s review of the lawfulness of detention, that a real prospect exists that the removal can be carried out successfully, having regard to the periods laid down in Article 15(5) and (6) of Directive 2008/115, for it to be possible to consider that there is a ‘reasonable prospect of removal’ within the meaning of Article 15(4) of that directive. 66 Thus a reasonable prospect of removal does not exist where it appears unlikely that the person concerned will be admitted to a third country, having regard to those periods. 67 Consequently, the answer to Questions 3(a) and (b) is that Article 15(4) of Directive 2008/115 must be interpreted as meaning that only a real prospect that removal can be carried out successfully, having regard to the periods laid down in Article 15(5) and (6), corresponds to a reasonable prospect of removal, and that that reasonable prospect does not exist where it appears unlikely that the person concerned will be admitted to a third country, having regard to those periods. Question 4 68 By this question the referring court asks essentially whether Article 15(4) and (6) of Directive 2008/115 allow the person concerned not to be released immediately, even though the maximum period of detention provided for by that directive has expired, on the grounds that he is not in possession of valid documents, his conduct is aggressive, and he has no means of supporting himself and no accommodation or means supplied by the Member State for that purpose. 69 It must be pointed out that, as is apparent in particular from paragraphs 37, 54 and 61 above, Article 15(6) of Directive 2008/115 in no case authorises the maximum period defined in that provision to be exceeded. 70 The possibility of detaining a person on grounds of public order and public safety cannot be based on Directive 2008/115. None of the circumstances mentioned by the referring court can therefore constitute in itself a ground for detention under the provisions of that directive. 71 Consequently, the answer to Question 4 is that Article 15(4) and (6) of Directive 2008/115 must be interpreted as not allowing, where the maximum period of detention laid down by that directive has expired, the person concerned not to be released immediately on the grounds that he is not in possession of valid documents, his conduct is aggressive, and he has no means of supporting himself and no accommodation or means supplied by the Member State for that purpose. Costs 72 Since these proceedings are, for the parties to the main proceedings, a step in the action pending before the national court, the decision on costs is a matter for that court. Costs incurred in submitting observations to the Court, other than the costs of those parties, are not recoverable.